[from a kid's sugar-enriched drink in fruity flavors] When someone who should know better succumbs to marketing influences and actually begins to believe the propaganda being dished out by a vendor, they are said to have drunk the Kool-Aid. Usually the decortication process is slow and almost unnoticeable until one day the victim emerges as a True Believer and begins spreading the faith himself. The term originates in the suicide of 914 followers of Jim Jones's People's Temple cult in Guyana in 1978 (there are also resonances with Ken Kesey's Electric Kool-Aid Acid Tests from the 1960s). What the Jonestown victims actually drank was cyanide-laced Flavor-Aid, a cheap knockoff, rather than Kool-Aid itself. There is a FAQ on this topic.

This has live variants. When a suit is blithering on about their latest technology and how it will save the world, that's ‘pouring Kool-Aid’. When the suit does not violate the laws of physics, doesn't make impossible claims, and in fact says something reasonable and believable, that's pouring good Kool-Aid, usually used in the sentence “He pours good Kool-Aid, doesn't he?” This connotes that the speaker might be about to drink same.